Betfair yesterday refused to settle its wagers after the top seed, Nikolay Davydenko, retired following irregular betting patterns on his tennis match at the Poland Open in Sopot. After winning his first set against the Argentinian Martin Vassallo Arguello 6-2, the Russian Davydenko lost the second 6-3 before withdrawing from the match with his opponent leading 2-1 in the third set.

The story of the match, coupled with the apparently unusual betting, prompted Betfair's market-integrity investigators to launch an investigation into the match. Davydenko and Arguello were unavailable for comment last night.

One punter who was following the match yesterday reported that, despite being the tournament favourite and No4 in the current ATP Tour rankings, Davydenko's pre-match odds drifted to 2.3, equivalent to an 11-8 against although he was set to face a player who stands 87th in the tour rankings. "Despite Davydenko winning the first set 6-2, Vassallo, an inferior player, was still favourite to win on the in-play Betfair market," said the punter.

When the match was suspended, Betfair was showing that $7,310,429 (£3,590,595 ) of bets had been placed and accepted on its site; this compared with approximately $3m during another second-round match at the same tournament, involving Steve Darcis and Tommy Robredo.

The online bookmaker is set to inform the ATP tour of its concerns over the match under the terms of its memorandum of understanding with the organisation, which has been in place since 2003.

It said in a statement: "Betfair has suspended settlement of the match-odds market on this afternoon's second-round match of the ATP Orange Prokom Open in Poland between Martin Arguello and Nikolay Davydenko, pending consultation with relevant regulatory authorities. Betfair has had a memorandum of understanding with the ATP since 2003 and will use it to exchange information should it become necessary."

A spokesman for the tour explained that it holds similar information-exchange agreements with other UK and European bookmakers. "The ATP takes issues surrounding gambling extremely seriously," said a spokesman. "We are committed to ensuring our sport remains corruption free and have strict rules in place governing this area.

"It would be inappropriate for us to comment further on any individual match or on the status of any potential investigation until such time as the process has been completed."

Betfair would not disclose how many times it had previously invoked its memorandum with the ATP tour, stating that such instances are never made public in any sport. However, it is now expected to broaden its investigation into other recent matches about which suspicions of impropriety have been raised. "We look at all matches all the time," said a spokesman for Betfair. "We have an integrity and fraud team of more than 40 people."

There will be particular attention paid to the concerns of punters on the Betfair site's tennis forum, one of whom told the Guardian: "It has become increasingly obvious recently that there are a significant number of 'fixed' tennis matches being played. It is obvious to anyone with some experience of the normal Betfair market behaviour and the appropriate odds for a tennis match that certain low-level ATP matches are being fixed, with corresponding irrational market patterns."

Betfair confirmed it would act on any such comments or debate on its forum. "We're all about transparency," added the spokesman. "You can't miss what is said on our forum, which is there for all to see."

How exchanges work

A betting exchange allows its clients to bet against each other by specifying the odds at which they are willing to bet on a particular event. They can either accept the best price being offered by other exchange users or specify a better price in the hope that another user will accept - or "lay" - the bet.

Alternatively, if they do not fancy the chances of a certain outcome, they can themselves lay the bets posted by other users. In a busy market the odds will often change by the second, particularly when an event, such as a horse race or a football match, has started and the betting is "in running".